The invention relates to a method for making a plastic film, the method comprising extruding a plastic film, mixing material into the plastic of the plastic film before extrusion, the material causing cavitation bubbles in the plastic film to be stretched, and orientating the plastic film by stretching after extrusion.
The invention also relates to an apparatus for making a plastic film, the apparatus comprising an extruder and at least one orientation device for orientating the extruded film.
The invention further relates to a plastic film which comprises bubbles with the maximum diameter of about 100 micrometers and the maximum height of about 10 micrometers, in which case the plastic film has been subjected to stretching and material which causes cavitation bubbles in the stretched plastic film has been mixed into the plastic of the plastic film.
Making a plastic film by extruding it and then orientating it is known e.g. from U.S. Pat. Nos. 3,244,781 and 3,891,374. It is, however, difficult to make thin and in particular thin foamed films using these solutions.
EP publication no. 0,182,764 discloses a thin polypropylene film which contains wide and flat disc-like bubbles, which are about 80 micrometers in length and about 50 micrometers in width. The film is produced by extruding material which has been foamed chemically or by means of gas and by orientating the extruded material biaxially. The result is a very versatile plastic film. However, the foaming degree of the film is less than 50%, which is why the properties of the film are not good enough for all purposes.
U.S. Pat. No. 3,634,564 discloses orientation of a foamed film to obtain a fiberized film. The foamed film is formed by mixing a foam forming substance into the plastic material. The mixture is extruded, which yields a foamed film, which is stretched. The bubbles of the film obtained are, however, rather large.
U.S. Pat. No. 4,814,124 discloses a film made of polyolefin and a filler which is stretched to obtain a gas permeable porous film. However, the foaming degree of such a film is not sufficiently good, nor are the mechanical properties of such a porous film sufficiently good for acoustic applications, for example.
Furthermore, it is not possible to produce thin films of polymethylpentene or cyclic olefin copolymer using the prior art solutions.
The object of this invention is to provide a very good and thin foamed plastic film and a simple and reliable method and apparatus for making said plastic film.
The method of the invention is characterized in that after orientation the plastic film is subjected to pressurized gas so that the gas diffuses in cavitation bubbles, and thus bubbles containing gas are formed in the plastic film.
The apparatus of the invention is characterized in that the apparatus comprises gas supply means arranged after at least one orientation device for feeding pressurized gas into the plastic film after orientation by stretching so that the fed gas diffuses in the cavitation bubbles that are formed in the plastic film during stretching, and thus bubbles containing gas are formed in the plastic film.
The plastic film of the invention is characterized in that the plastic film is subjected to the pressure of pressurized gas after stretching so that the bubbles contain said gas and the foaming degree of the plastic film is over 70%.
The basic idea of the invention is that a film is extruded from plastic material by means of an extruder and material has been mixed into the plastic so that when the plastic is stretched cavitation bubbles are formed in the material particles mixed into the plastic. The film is orientated by stretching and after that gas is fed into the film under high pressure so that the gas diffuses in the cavitation bubbles and causes overpressure in them. The idea of a preferred embodiment is that after the first orientation and feeding of gas the plastic film is orientated by stretching it in the direction substantially perpendicular to the first orientation direction, and thus the overpressure is released in the cavitation bubbles and the bubbles expand.
An advantage of the invention is that very thin films with a foaming degree of about 70 to 90% can be provided in a relatively simple manner. An advantage of the high foaming degree is that the electric and mechanical properties of the film are very good. Since the film becomes thinner as it is orientated by stretching, gas diffuses in the orientated film substantially faster than in an non-orientated film, i.e. gas can be fed into the film particularly efficiently by not feeding gas which acts on the film until after orientation. A further advantage is that the method and apparatus can be used for making a film for example of polymethylpentene or cyclic olefin copolymer or a combination thereof.